Drop UUID for blocks in favor of short codes

@kennethaar updating potentially tons of block references every time that a block is moved is not feasible. Imagine adding a block at the top of many blocks that are referenced somewhere else: each referenced block is moved, its number change and so every reference must be updated. And this would work only in Logseq, that would care of updating the references, and editing made in other apps would break the references for the mechanism I just described.